In the manufacture of semiconductor devices, one of the most important and fundamental requirements of such devices is that they be formed from precursor materials which have particular properties. In many applications, the appropriate precursor is a single crystal (sometimes referred to as "monocrystalline") material. As is known to those familiar with electronic devices, if the growth and processing of a crystal are not carefully controlled, defects will occur in the crystal which will substantially affect the electronic properties of the crystal and of any resulting device made from it. In many cases defective crystals render such devices inoperable.
One of the distinguishing features of crystals is their internal order. This internal order includes a regular repetition of a structural pattern of species (e.g. atoms, ions or molecules) usually in three dimensions. This regular repetition extends not only to the immediate environment of each particular atom, ion or molecule, but also through large distances representing millions of repetitions of the basic structural pattern.
One disadvantage of such repetition in the crystal structure, however, is that whenever a defect occurs in the crystal structure, the nature of crystal growth is such that the defect in the crystal will likewise be repeated over and over again as the crystal grows. These defects are often called dislocations and such structural imperfections typically extend for distances of hundreds of Angstroms. Typical types of dislocations are referred to as screw dislocations, edge dislocations, stacking faults, antiphase boundaries, and double positioning boundaries. These defects can be severe enough to appear as visible nonuniformity in the surface morphology of the crystal. Additionally, in a material such as silicon carbide that can form in a large number of polytypes separated by small thermodynamic differences, such defects encourage the undesirable nucleation of polytypes other than the polytype of the base crystal.
Various negative electrical effects typically result from such defects and dislocations. The most serious electrical effect is the appearance of undesirable or unacceptable amounts of leakage current in many types of devices. Another effect is lowered electron mobility resulting from scattering collisions in defective crystals. In turn, the lowered electron mobility exhibits itself as an increased resistance in the particular device. In optical devices such as light emitting diodes, crystal defects cause changes in the emitted wavelength and encourage unacceptably broad bandwidths in the emissions.
One crystal growth technique which is desirable in the manufacture of certain semiconductor devices and necessary in the manufacture of others is the growth of thin films of a monocrystalline semiconductor material upon another surface of the same or another semiconductor material, or even upon a non-semiconducting substrate. Such growth is usually accomplished by techniques such as liquid phase epitaxy or chemical vapor deposition and those of ordinary skill in the semiconductor arts are generally familiar with them. These processes generally proceed as a careful building up of a thin film or "epitaxial" layer of new crystalline material upon the existing material. The nature of crystal growth processes are such that defects in the surface upon which the new crystalline material is to be deposited will tend to be faithfully repeated in the new crystalline material which grows upon it. Because the epitaxial layers are typically those portions of the semiconductor material that are used to form and operate the desired electronic devices, these repeated defects can make the resulting crystal less desirable or even nonfunctional from an electronic standpoint.
There thus exits the need to properly prepare surfaces upon which epitaxial crystal growth can take place while minimizing or substantially eliminating defects during the production of semiconductor material for electronic devices.
The problem with all surface preparation techniques, however, is that of attempting to remove existing damage or defects from the surface without causing further damage in the attempt. In this regard, those familiar with the preparation of semiconductor surfaces will recognize that a typical technique includes producing a bulk crystal, sawing or slicing the crystal into smaller crystals or wafers, lapping the crystal by using an abrasive paste in conjunction with a hard surface to quickly remove a fairly large amount of material, and polishing the crystal surface by using a similar paste, but in a milder fashion and with a softer surface to produce a finished surface of semiconductor material. In each case, these steps form "damage nucleated defects" which are reproduced during the epitaxial growth process.
In the case of polishing silicon (Si) wafers, damage-nucleated defects can be easily removed in a final polishing step by using a "polishing etch." Such a step uses a very fine abrasive in a polishing medium that also chemically etches the Si surface as it is being polished. The resulting surface is both smooth and substantially free of damage. Typical polishing etches for Si use a suspension of SiO.sub.2 particles in a basic solution (NaOH or KOH) that oxidizes the Si surface. This oxide is in turn mechanically removed by the SiO.sub.2 particles.
As set forth in U.S. Pat. No. 4,865,685, interest has recently been rekindled in the use of silicon carbide (SiC) as a semiconductor material. Silicon carbide has long been recognized as having certain favorable characteristics as a semiconductor material. These include a wide band gap, a high thermal conductivity, a high saturated electron drift velocity, and a high electron mobility. As obstacles to its commercialization, however, silicon carbide requires high process temperatures for otherwise ordinary techniques, good starting materials are difficult to obtain, certain doping techniques have traditionally been difficult to accomplish, and perhaps most importantly, silicon carbide crystallizes in over 150 polytypes, many of which are separated by very small thermodynamic differences. Nevertheless, recent advances, including those discussed in U.S. Pat. Nos. 4,865,685 and 4,866,005 have made it possible to produce silicon carbide and silicon carbide based devices on a commercial basis and scale.
In accomplishing epitaxial growth of silicon carbide, however, it has been found that the usual techniques of slicing, lapping and polishing silicon carbide surfaces followed by chemical vapor deposition (CVD) produces a number of apparent defects including stacking faults and the like. These apparently result from subsurface damage caused by these mechanical preparation steps, and that the follow-up steps such as fine polishing, wet etching and oxidation do not repair. These in turn lead to the repetitive defects as the epitaxial layer is grown upon the surface.
Additionally, preparation steps that are suitable for materials such as silicon are not analogously suitable for silicon carbide. As stated earlier, silicon surface preparation uses a chemical polish that slightly etches the silicon chemically, as well as physically. Silicon carbide is, however, much more chemically stable--and thus nonreactive--than silicon and no suitable analogous chemical polish is presently known or used with silicon carbide. As a result, different techniques are used with silicon carbide.
One technique for removing remaining defects from SiC surfaces a liquid or "wet" etch using molten salt materials such as potassium hydroxide (KOH). Molten salt etches are hard to control, however, require generally high temperatures (e.g. 700.degree. C.-800 .degree. C.) and tend to be chemically aggressive and hard to control.
Another attempted solution includes anodic etching, in which the sample to be etched is used as the anode in an electrolytic circuit using relatively concentrated potassium hydroxide (e.g. 20% aqueous KOH), chromic acid, or oxalic acid as the electrolyte. This has the disadvantage, however, that under any given set of conditions, p and n type material will etch at significantly different rates. Additionally, anodic etching is a low volume process which is often commercially unsuitable, and electrolytic plating also becomes a problem under certain conditions.
An additional technique of SiC surface preparation is oxidation of a surface followed by removal of the oxidized portion. Oxidation techniques, however, have numerous problems including the failure to remove enough material at a viable rate. For example, it can take up to a week of oxidation growth and removal to remove one micron of certain types of materials. Oxidation techniques can also result in dopant re-distribution effects, and oxidationinduced stacking faults.
Accordingly, there exists a need for a method of preparing silicon carbide surfaces for further crystal growth, particularly epitaxial growth, that removes the damage caused by the necessary cutting and polishing steps, while avoiding introducing additional defects that result from the preparation step itself.